To minimize the risk of a call being cut off due to the unavailability of radio resource in the various cells to which a call may be transferred, also known as "candidate" cells, it is known to provide a special procedure called "preemption" for certain priority users, such as the police, the fire brigade, etc. In the event that no resource is available in a particular candidate cell (the "target" cell) towards which a transfer of a priority user has been requested, preemption consists purely and simply in taking a resource that has already been allocated to a non-priority user in the target cell. The call being made by the non-priority user is then either cut off or else transferred to a cell in which resource is available, but with the attendant risk of degraded quality.
A preemption procedure of this kind is provided in the cellular radio system for communication with mobiles that is known under the initials GSM (for "global system for mobile communications").
It is briefly recalled that a system such as the GSM system comprises, as shown in FIG. 1;
a set of base transceiver stations (BTS) such as those referenced BTS1, BTS2, BTS3, and BTS4, each of these base stations being allocated a cell (referenced respectively C1, C2, C3, and C4), and said base stations being in communication with mobiles, such as M1, M2, M3, and M4 situated in said cells; PA1 a set of base station controllers (BSC) such as those referenced BSC1 and BSC2, with each BSC concerning a subset of the base stations, i.e. of the cells, specifically with respect to resource management, the base station controller BSC1 controlling base stations BTS1 and BTS2, i.e. cells C1 and C2, in the present example, while the base station controller BSC2 controls base station BTS3 and BST4, i.e. cells C3 and C4; and PA1 a set of mobile switching centers (MSC) such as the center referenced MSC1, with each of the MSCs controlling a subset of base station controllers, particularly from the call management point of view, the mobile switching center MSC1 in the example shown controlling base station controllers BSC1 and BSC2. PA1 the server BSC, written BSCs, begins by sending a handover request message (HO REQ) to the MSC; PA1 in turn the MSC sends a message of the same type (HO REQ') to the target BSC; PA1 depending on whether or not resource is available in the target cell, the target BSC sends to the MSC either a handover command (HO COMM) message, as shown in FIG. 2, or else, a message indicating that it is not possible to make a handover to the target cell in question (HO FAIL) as shown in FIG. 3; and PA1 the MSC in turn sends a message of the same kind to the sever BSC, i.e. either a handover command message (HO COMM'), or else a rejection message indicating that it is not possible to make a handover to the target cell in question (HO REJ).
In the event of a request being made to transfer a call from a server cell to a target cell under the control of the same BSC as the server cell, e.g. in the event of a request to transfer a call from cell C1 to cell C2, or from cell C3 to cell C4 (with such a transfer being known as an intra-BSC handover), the BSC in question is in a position on its own to determine whether resources are available in the target cell, and where appropriate, for a priority user, to implement internally the preemption procedure for allocating resource to the user.
When there is a request to transfer a call from a server cell controlled by one BSC, referred to as the "server" BSC, to a target cell controlled by another BSC, known as a "target" BSC, e.g. in the event of a request being made to transfer a call from one or other of the cells C1 and C2 to one or other of the cells C3 and C4 (with such a transfer also being known as an "inter-BSC handover"), the procedure for requesting transfer is as shown in one or other of FIGS. 2 and 3, namely:
When no resource is available in the target cell, and the mobile concerned is a priority mobile, then the above procedure is modified, as shown in FIG. 4, by the MSC sending the target BSC a request to implement the preemption procedure in its message HO REQ', with this being done by means of a special preemption capability indicator (PCI) bit (see in particular page 83 of GSM Recommendation 08.08, Version 5.2.0., May 1996), and by the fact that the target BSC then implements said preemption procedure to find resource in the target cell, such that the message HO FAIL and HO REJ are replaced by handover command messages (HO COMM and HO COMM').
Thus, in the context of an inter-BSC transfer request, the MSC performs a request to implement the preemption procedure immediately on receiving a handover request message and merely on recognizing the priority status of the mobile in question (where such priority status is included in the subscription data relating to the mobile, and the MSC has access to said data). Under such circumstances, preemption is performed in the target cell constituted by the first of the candidate cells to which a transfer has been requested, even when it is possible that available resource exists in one or more candidate cells other than said candidate cell, or even in said candidate cell itself. This penalizes non-priority users in pointless manner. In such systems it is important not only to keep down the risk of a call being cut off for priority users, but also to avoid excessively penalizing non-priority users whenever the preemption procedure is implemented.